A wide variety of optical systems include an imaging sensor that typically includes an array of photo-sensitive detectors, often termed a focal plane array (FPA). Each detector produces an output that corresponds to a pixel in the image produced by the imaging sensor. Each detector generally includes a photo-diode and electronic components for measuring the intensity of received light. An array may include many hundreds or thousands of detectors. Each detector in the array may have a slightly different sensitivity or response to same received light (i.e., light with the same wavelength, intensity, etc.). This non-uniform sensitivity yields Fixed Pattern Noise (FPN) in the images produced by the detector array. The fixed pattern noise causes some pixels in the image to be too bright, while others are too dark. This negatively impacts the Signal to Noise Ratio (SNR) of the imaging sensor, and the guidance system of a seeker when the imaging sensor is used to guide a projectile to its intended target. Therefore, to be able to produce accurate, high resolution images and accurately track a target in a seeker, it can be important to compensate for the non-uniformity in the detector outputs across the array.
A Non-Uniformity Compensation (NUC) system adjusts each pixel gain and offset to compensate for the FPN by applying a unique correction to each pixel. Typically a Two Point Non-Uniformity Compensation (Two Point NUC) is done on the ground prior to flight to make the corrections at two specific scene temperatures. However, if during flight the sensor is exposed to background scenes that are far from the corrected temperatures, the system noise will rise in proportion to the background noise. Therefore, in situ, real time, scene based NUC correction during flight may be necessary. An Adaptive Non-Uniformity Compensation (ADNUC) system adjusts each pixel dynamically to compensate for the differing sensitivity of each detector in the FPA.
Non-uniformity correction can be done by masking the array during a calibration procedure, such that every detector has the same, known input, and measuring the output from each detector. Differences in the outputs can be used to provide gain and offset calibration coefficients that are applied, for example, by altering the bias voltages at each detector, or during image processing, such that for the same known input to every detector, every pixel in the image has the same color and intensity. Conventionally, masking the array is done by placing a light-blocking shield over the array to prevent the detectors from receiving light, and the outputs are measured in this “dark state.”
Non-uniformity correction is often needed in optical systems that operate in the visible spectrum (e.g., imaging systems that produce color images of a viewed scene) or the infrared spectrum (e.g., seekers or other thermal imaging systems). In seekers, non-uniformity correction is typically done using an opaque (light-blocking) paddle that is driven by a mechanical servo to move the paddle into and out of the optical path or field-of-view of the imaging sensor. However, the need to move the paddle into and out of the field-of-view of the sensor requires bulky moving parts, and this type of mechanism can be difficult to incorporate into systems with constrained packaging requirements.